How to Limit the Backlog Period in SNP

Hi ,
Please advise me to limit the the period of backlog order in SNP. In our scenario system considers the backlog in forecast/sales order for the future requirement and there is no limit to it. We want to limit it for one month ,i.e, system should consider the backlog in sales order/forecast only for one month should not consider the other month. How can we implement it. Please advise.
Regards,
Vijesh